1. Field of the Invention
The invention relates to an extruder temperature controller and a method for controlling the temperature of an extruder barrel zone of an extrusion device. Specifically, the invention relates to an extruder temperature controller and a method for controlling the temperature of an extruder barrel zone wherein the control of the temperature is responsive to a change in the speed of an extruder screw.
2. Description of the Background Art
Extrusion devices are often used in the plastics or other industries to continuously melt, blend, form, and solidify plastics or other extrusion materials into a desired shape. Typical extrusion devices include a rotating screw housed coaxially within a heated, cylindrically-shaped barrel. The screw rotates within the barrel and drives an extrusion material such as plastic through the barrel. The extrusion material is forced through a die or aperture at the end of the barrel. A temperature drop, that occurs when the extrusion material leaves the heated barrel, allows the extrusion material to solidify in a molded shape that is determined by the profile of the die.
The temperature of the extruder barrel zone or "heat exchange zone" is one variable that can be controlled during an extrusion process. The control of the barrel temperature, therefore, eliminates a variable while extruding material such as plastic. The extruder barrel can be operated to control the temperature of the extrusion material within the barrel under one or more of three conditions. An extruder barrel can (1) increase the temperature of an extrusion material, (2) decreases the temperature of an extrusion material, or (3) maintain the temperature of an extrusion material. The third condition of maintaining a temperature of an extrusion material occurs when an extruder is operated at a speed wherein the heat gain from the friction of the extrusion material coming, as the extrusion material is processed in the extruder barrel, is approximately equal to the heat loss from the extruder barrel. This condition of no heat gain or loss in known as an "adiabatic" condition.
Most extrusion devices have a plurality of heat exchange zones. The temperature of each heat exchange zone can be independently controlled. This independent control is such that one or more heat exchange zones heat the material, that is being processed, while the remaining heat exchange zones are in an adiabatic condition or are cooling the extrusion material. It is common for a heat exchange zone near the die end of an extruder barrel to be used to maintain the temperature of an extrusion material which would otherwise be increased as the extrusion material passed through the barrel just before the extrusion material is extruded through the die. This procedure indirectly cools the extrusion material. An extruder barrel, typically, has five to eight heat exchange zones, but the number of heat exchange zones can vary and often depends upon the size of the extruder.
An extruder temperature controller can control the temperature of its extruder barrel with heat exchange elements. The extruder barrel is, commonly, surrounded by a shell containing heat exchange elements. The heat exchange elements can be (1) heaters such as resistive heaters which increase the extruder barrel temperature and (2) cooling tubes for circulating water or another coolant in order to remove heat and maintain the temperature at a desired setpoint for the extruder barrel temperature. Alternative heat exchange elements can be used. For example, the cooling structure can be a finned shell with a blower that circulates air past the fins.
Temperature sensors, such as thermocouples, are positioned in extruder barrels to signal the temperature at the location of the sensor. Two thermocouples per extruder barrel zone can be provided and are electrically isolated from one another. A first thermocouple is known as the "A thermocouple" of the pair and is placed juxtaposed to the inner surface of the extruder barrel. The end of the "A" thermocouple is usually in contact with an inner liner of the extruder barrel. A second thermocouple is known as the "B thermocouple" of the pair and is placed in the interior of the heater/cooler shell. Each heat exchange zone of the extruder is similarly provided with a pair of thermocouples, "A" and "B", similarly placed. An air-cooled extruder system also has the B thermocouple in the interior of the shell.
An extruder temperature controller receives signals from the temperature sensors. The extruder temperature controller determines whether the temperature of a given heat exchange zone is too cool or too hot and, if necessary, signals the appropriate heat exchange elements to increase or decrease the temperature in the particular heat exchange zone regulated by that controller.
The extruder barrel and the heat exchange elements store residual heat. Due to the mass and thermal conductivity of the composition and the extruder barrel, a delay in the transfer of heat occurs resulting in a lag between the actual temperature and the signalling of instructions for that temperature by the extruder temperature controller to increase or decrease the temperature of a heat exchange zone. For example, when the extruder temperature controller instructs a heating element to cease applying heat, energy stored in the heating element continues to warm that heat exchange zone of the extruder barrel. This continued warming causes the extruder barrel temperature to continue to rise in that heat exchange zone. The lag between the issuance of an instruction from the extruder temperature controller and the response from the heat exchange elements causes the extruder barrel temperature to oscillate about the desired temperature.
U.S. Pat. No. 3,866,669 to Gardiner and U.S. Pat. No. 3,751,014 to Waterloo both address the problem of oscillating extruder barrel temperatures. In the systems described in Gardiner and Waterloo, a first temperature probe or thermocouple provides a "deep" temperature measurement representative of the temperature of the extrusion material. A second thermocouple is positioned within the shell surrounding the extruder barrel to provide a "shallow" temperature measurement representative of the temperature of the heat exchange elements. The electrical signals from the pair of thermocouples are combined to provide an average value. The extruder temperature controller monitors the weighted average value and selectively activates the heating and cooling elements to maintain the average value at a temperature that is approximately equal to a setpoint representative of the desired temperature for the extruder barrel zone.
The control of the heat exchange elements by an extruder temperature controller that is responsive to an average value for zone, that is being processed, reduces temperature and/or control signal oscillations. An example of such a temperature oscillation occurs during operational conditions wherein a resistive heating element applies heat to increase the temperature of an extruder barrel. While the heating element is active, the shallow temperature measurement is higher than the deep temperature measurement. This temperature difference occurs because the shallow temperature probe is positioned in the vicinity of the activated heating element. Accordingly, the average value of the extruder temperature controller is also greater than the deep measurement or the actual temperature of the extruder barrel zone. The average value reaches the temperature setpoint while the actual temperature of the extruder barrel zone is still below the desired temperature. The extruder temperature controller inactivates the heating element after the average value reaches the temperature setpoint, but before the extruder barrel zone reaches the desired temperature. The heat stored in the heating element continues to raise the temperature of the extruder barrel zone toward the desired temperature. Such temperature oscillations can also occur during operational conditions wherein the temperature of the extruder barrel zone is being decreased.
Inactivating the heat exchange elements before the extruder barrel zone has reached the desired temperature prevents the temperature of the extruder barrel zone from "overshooting" the desired temperature which can cause undesirable temperature oscillations. This advantage is achieved at the expense of a reduction in the accuracy with which the temperature of the extruder barrel zone is controlled. More specifically, since the extruder temperature controller operates to correct the temperature only when the average temperature value deviates from the desired temperature, the extruder temperature controller may not attempt to adjust the temperature, even when the temperature of the extruder barrel zone remains below a desired elevated temperature or above a desired cooling temperature.
U.S. Pat. No. 31,903 to Faillace describes an extruder temperature controller which anticipates changes of temperature in an extruder barrel. This system monitors a control sum error to determine when the temperature has not changed significantly for a specified length of time or when the system has "stabilized". Once the system has stabilized, this extruder temperature controller examines the actual temperature of the extruder barrel zone as indicated by the deep measurement and compares the actual temperature to the desired temperature. If the actual temperature is significantly different from the desired temperature, this extruder temperature controller calculates and changes the temperature setpoint so that the control sum error appears to require a temperature adjustment. If the actual extruder barrel zone temperature is, for example, too low, the Faillace extruder temperature controller raises the setpoint above the desired temperature. The control sum error is then below the setpoint, which causes the extruder temperature controller to adjust the temperature until the control sum error is minimized.
Changes in the rotational speed of the extruder screw or "screw speed" are normal during the start-up and the shutdown of an extrusion line. However, screw speed changes typically cause a thermal load variation which is troublesome in an extrusion process. An example of this condition occurs in blow molding processes wherein the molded piece becomes jammed when exiting the mold. Sensors, which detect the jammed piece, rapidly shutdown the extruder system in order to prevent further jams and potential damage to the mold system. The extruder system during normal operation in a blow molding process runs at a preset speed.
The extruder temperature controller of the Faillace Reissue Patent in a blow molding process resolves a reset value for each heat exchange zone. The reset value is proportional to the temperature offset for that heat exchange zone, which is proportional to the thermal load for that heat exchange zone. The Faillace extruder temperature controller resolves a reset value for each heat exchange zone, individually. The criteria for a heat exchange zone to reset is as follows:
(1) the reset is enabled; PA0 (2) the heat exchange zone is in stable control and no control alarms occur for a period of time such as a control stability time; PA0 (3) the offset is greater than the actual alarm band which, typically, means that one offset is greater than 3.degree. F. (1.6.degree. C.); PA0 (4) the reset has not occurred within the reset stability time; and PA0 (5) the reset limit is not reached.
When an extruder system for a blow molding process is stopped due to a jam, it is typically restarted within a few minutes. The minimum time a heat exchange zone must be stable in control or "minimum reset stability time" is approximately 4 minutes. The actual time during which a heat exchange zone recovers from a step change in load, such as a sudden stop condition, is approximately 10 to 12 minutes. Therefore, the reset means in the Faillace extruder temperature controller cannot respond quickly enough to compensate for a step change in load which lasts for less than 10 to 12 minutes. The result of this condition is that a heat exchange zone is offset in temperature equal to the difference in thermal load at the normal running screw speed compared to the screw speed at stop. In addition, if the extruder system remains stopped for a period of time which allows the reset to actuate, such as when a jammed piece is cleared and the extruder system returns to a normal operating screw speed, the incorrect heat exchange zone temperature reset value causes a temperature offset. This temperature offset remains until a reset value can be resolved at the normal screw speed and compensates for the thermal load at that screw speed. This condition in a blow molding process causes a significant change in the characteristics of the plastic melt output of the extruder system. These changes cause a variation in the weight of the blow molded products. This variation can degrade the quality of the end product by causing variations in the wall thickness of the product. These variations in quality cause waste, inefficiency, and undue expense.
The Faillace extruder temperature controller does not eliminate temperature control problems in other process applications which are similar to the blow molding process. Such process applications include wire and cable coating processes. The splicing of a bare conductor or the changing of reels, while the extruder is running, during these processes produces the same temperature offset problem described above. The wire and cable processes, during a cable splice or reel change, require the extruder system and the entire production line to be slowed from a normal run speed to a speed which facilitates the splice or reel change. This slowing of the extruder screw speed causes a significant thermal load difference between normal running speed and the reduced running speed of the extruder system. The end result of this situation can be a significant change in the characteristic of the plastic melt output from the extruder system that is being applied as coating to the wire or cable. This change potentially degrades the quality of the end product.
The industry is lacking an extruder temperature controller for an extruder system that preempts a temperature reset value for each heat exchange zone upon a change in the screw speed of the extruder system.